Pyrotechnic means for interrupting the light output of a flash bulb

ABSTRACT

IN A PHOTOFLASH SYSTEM, MEANS ARE PROVIDED FOR CONTROLLING THE EFFECTIVE LIGHT OUTPUT FROM A FLASH BULB IN ACCORDANCE WITH DEMAND. THE MEANS INCLUDE LIGHT SENSING MEANS RESPONSIVE TO LIGHT REFLECTED FROM THE OBJECT OF THE PHOTO. SIGNALS FROM THE SENSING MEANS ARE INTEGRATED AND USED TO TRIGGER MEANS FOR OBFUSCATING THE LIGHT SOURCE. THE LATTER MEANS MAY INCLUDE A VAPORIZABLE SUBSTANCE, A DEFLAGRATING SUBSTANCE OR THE LIKE WHICH, IN RESPONSE TO THE TRIGGER SIGNAL, FORMS AN OPAQUE SHIELD BETWEEN THE LIGHT SOURCE AND THE PHOTOGRAPHIC OBJECT.

H. OWENS Re. 27,421 PYROTECHNIC MEANS FOR INTERRUPTING THE LIGHT OUTPUTOF A FLASH BULB Original Filed Jan. 26, 1968 July 4, 1972 4 Sheets-Sheet1 I INVENTOR. W l L E Y OWENS ATTORNEY.

y 4, 1972 w. H. OWENS Re. 27,421

PYROTECHNIC MEANS FUR INTERRUPTING THE LIGHT OUTPUT OF A FLASH BULBorlginal Filed Jan. 26, 1968 4 Sheets-Sheet 2 FIG. 6 6

l I! I I NORMAL FLASH BULB I t LIGHT OUTPUT I g 4 u: I E l g r LlGHTOUTPUT WITH ,1 CONTROL SHUTTER 52 5 l I 3O 4O TIME (MILLISECONDS) FIG. 8

INVENTOR. WILEY OWENS ATTORNEY.

July 4,1972 H. OWE I R 27,421

PYROTECHNIC ME FOR INTE PTING THE LIGHT UTPUT OF A FLASH BULB OriginalFiled Jan. 26, 19 4 Sheets-Sheet 3 F l G. B 84 7O .rramlllllllllNVENTOR. wu. OWENS ATTOR N EY.

I w. H. OWENS Re. 27,421 PYRO'I'ECHNIC MEANS FOR INTERRUPTING THE LIGHTOUTPUT OF A FLASH BULB July 4, 1972 4 Sheets-Sheet 4 Orlginal Filed Jan.26, 1968 INVENTOR.

WILEY OWENS ZiW M ATTORNEY.

United States Patent Int. Cl. G01j 1 20 US. Cl. 250-201 Claims Matterenclosed in heavy brackets [II appears in the original patent but formsno part of this reissue specification; matter printed in italicsindicates the additions made by reissue.

ABSTRACT OF THE DISCLOSURE In a photoflash system, means are providedfor controlhug the effective light output from a flash bulb inaccordance with demand. The means include light sensing means responsiveto light reflected from the object of the photo.

Signals from the sensing means are integrated and used to trigger meansfor obfuscating the light source. The latter means may include avaporizable substance, a deflagrating substance or the like which, inresponse to the trigger signal, forms an opaque shield between the lightsource and the photographic object.

In the art of flash photography, means have heretofore been providedwhereby, in the case of electronic flash devices, the time duration ofthe flash is controlled in accordance with the demand as determined bythe film speed, diaphragm opening and distance to the photographicobject. Those devices actually quench the flash tube by sh0rtcircuitingthe flash tube energy source, thereby assuring that the proper amount oflight for a good picture is obtained, minimizing the probability ofover-exposure.

On the other hand, most tyro photographers use the more conventionalflash bulbs. While many efforts have been made to make photography forsuch users as simple as possible, no comparable means have been providedfor bulb-type flash devices. Such tyro photographers are those most inneed of aids in making acceptable photographs.

Accordingly, it is an object of the present invention to provide animproved photoflash control means which obviates the foregoingshortcomings.

It is another object of the present invention to provide an improvedphotoflash device as set forth which is usable with conventional flashbulbs.

It is a further object of the present invention to provide a flashcontrol means for bulb-type flash devices wherein auxiliary means areprovided for effectively cutting off the light from the source after atime interval determined by the demand of the conditions.

In accomplishing these and other objects there has been provided, inaccordance with the present invention, in association with aphotographic camera, a photoflash apparatus including flash bulb meansfor illuminating an object to be photographed. There is also providedmeans for sensing the light reflected from the object to the camera,producing a corresponding signal. The latter signal is integrated toproduce a control signal. The control signal is, in turn, used tocontrol the actuation of a light cut-off means. The latter means mayinclude means within a specially constructed flash bulb which iseffective instantly to blacken the walls of the flash bulb envelope.Alternatively, the latter means may include means external to the flashbulb, but interposed between the flash bulb and the object. The externalmeans then includes a normally transparent member and meanssubstantially instantly to respond to the control signal to render ther. CC

transparent member opaque. In both of the foregoing alternatives, thecut-off means effectively comprises a signal responsive shutter meansfor automatically controlling photographic exposure with illuminationfrom flash bulbs while taking ambient light conditions intoconsideration.

A better understanding of the present invention-may be had from thefollowing detailed description when read in connection with theaccompanying drawings, in which:

FIG. 1 is a schematic illustration of a photographic system embodyingthe present invention;

FIG. 2 is a schematic circuit diagram of a control circuit useful in thepractice of the present invention;

FIG. 3 is a representation of a flash bulb with one form of shuttermeans in accordance with the present invention;

FIG. 4 is a representation of a flash bulb with another form of shuttermeans in accordance with the present invention;

FIG. 5 is a representation of a flash bulb with a further form ofshutter means in accordance with the present invention;

FIG. 6 is a graph illustrating the operation of the control shutter inaccordance with the present invention; and

FIGS. 7 through 15 illustrate the various steps in the production of apreferred form of the shutter means in accordance with the presentinvention.

Referring now to the drawings in more detail, there is shown in FIG. 1,a camera 2 with which to take a photograph of an object 4. The camera 2is of the type adapted to accommodate suitable photoflash apparatus 6such as the illustrated flash cube. The so-called flash cube is aunitary structure generally in the form of a cube and including a flashbulb and reflector in each of the four vertical faces. These cubes arearranged such that when properly mounted on the camera, one of thevertical faces is aimed at the same direction as the lens of the camera.Upon actuation of the shutter mechanism of the camera, power is suppliedto the flash cube from a power supply contained within the camera tocause the flash bulb in the front face of the cube to be flashed insynchronism with the operation of the camera Shutter. Under ordinarycircumstances, the light from the flash cube 6 would illuminate theobject 4 while the shutter of the camera 2 is opened. Light reflectedfrom the object 4 passes through the lens of the camera 2 and exposesthefilm within the body of the camera. Interposed between the flash cube 6and the object 4 there is a shutter 8. The shutter 8 is, by means whichwill be hereinafter described in detail, operative under controlconditions to interrupt the flow of light from the flash cube to theobject when a predetermined amount of light has been reflected by theobject back to the camera. In order to effect the desired control overthe actuation of the shutter 8, a light responsive control means 10 isprovided. The control means 10 includes a suitable photocell 12, orother suitable light detecting instrumentality, connected in anelectronic circuit which is adapted to produce a control signal inresponse to the integral of the amount of light detected by thephotocell to actuate the shutter 8.

In operation, the photographer actuates the shutter of the camera 2 inorder to take a photograph of the object 4. In the usual manner, theoperation of the camera shutter closes a synchronizing switch whichallows the flash bulb 6 to be energized from a power Source con tainedwithin the body of the camera. The light emitted from the flash bulb 6passes through the framework of the shutter 8 toward the object 4. Lightreflected from the object 4 not only passes through the lens of thecamera onto the film therein, but also onto the photocell 12 of thecontrol means 10. Within the control means 10 there is provided suitablecircuitry for integrating the signal produced by the reflected light onthe photocell 12. When the integrated signal reaches a predeterminedvalue, the

3 ontrol means produces a trigger signal which actuates he shutter 8.Actuation of the shutter 8 produces a subtantially instantaneousopacity, blocking the passage of ight from the flash bulb 6 toward theobject 4. The preletermined value at which the trigger signal isproduced established at that level whereat sulficient light has :eenreceived at the camera to produce a proper exposure it the film withinthe camera. The value is, of course, L function of the film speed andthe aperture, or lens npening. Since the light quantity detected by thephoto- :ell 12 and measured by the control means is an abolute value,the photographer need not be concerned vith camera to object distance(within the normal range if the flash bulbs), the ambient light, nor thereflecivity of the object. Most of the cameras designed for use )y theclass of photographers for which the present inention is intended have apreset lens opening. The only 'ariable remaining for which the tyrophotographer must nake adjustments in the control means of the presentnvention is film speed.

In FIG. 2, there is shown a schematic representation of t circuitsuitable for use in the control means d0. This :ircuit includes a powersupply 14- which, for example, nay be the power supply or battery packcontained withn the body of the camera and which is also used to fire hefiash bulb 6. The photocell 12 is illustrated as a photoensitivetransistor in a so-called Darlington configuration. lhis photocell ischaracterized in that the current flowing hrough the cell is controlledin accordance with the mount of light impinging thereon. The collectorof the )hOtOS6IlSltlVfi transistor combination is connected to theositive terminal of the power supply 14. The emitter of hephotosensitive transistor 12 is connected through an ntegratingcapacitor 16 to the other terminal of the ower supply *14. A switch 18is connected across the :lectrodes of the integrating capacitor 16. Basebias for he transistor 12 is provided by an adjustable resistor 20:onnected in series between the base electrode of the tranistor 1'2 andthe negative side of the power supply 14. I filter capacitor 22 isconnected across the resistor 20. the emitter of the transistor 12 isconnected through a Zener diode 24 to the gate electrode of a siliconconrolled rectifier 26. A bias resistor 28' is connected beween the gateelectrode of the silicon controlled rectifier ,6 and the negative sideof the power supply. The cathode if the silicon controlled rectifier '26is also connected to he negative side of the power supply. A blockingresistor l0 is connected between the positive side of the power suplyand a first electrode of a storage capacitor 32; the vther electrode ofthe capacitor 32 is connected to the regative side of the power supply14. A pair of output erminals 33 are connected, respectively, to theanode of he silicon controlled rectifier and the junction between heresistor 30 and a storage capacitor 32. Across these nutput terminals 34there is connected the shutter means i which is here shown as a fusiblemetallic element.

In operation, the switch 18 is normally closed short ircuiting thecapacitor .16. This switch may be synchroiized to open with the shutterof the camera. As the cam- :ra shutter opens, the flash bulb is fired,illuminating the vbject of the photograph. Light reflected from thatobject alls upon the photocell 12 causing current to flow through hecell 12 in accordance with the intensity and duration f the lightincident thereon. This current flowing through he cell 12 charges thecapacitor 16. As the capacitor 16 :harges, the voltage thereacrossincreases until it reaches he breakdown potential of the Zener diode 24.When he Zener diode 24 breaks down, a signal is applied on he gateelectrode of the silicon controlled rectifier 26.

In the static condition, the capacitor 32 is fully charged rorn thepower supply 14 through the resistor 30. As the riggering signal isapplied to the gate electrode of the ilicon controlled rectifier 26, thesilicon controlled recifier is caused to conduct. The conduction of theSCR 26 :auses the capacitor 32 to be discharged through the shuttermember 8. That current is shutter member 8.

It should be apparent that a predetermined amount of light reflectedfrom the object of the photograph is necessary to effect a triggering ofthe silicon controlled rectifier. This quantity of light is, of course,correlated with the amount of light necessary to properly expose thefilm in the camera. Since various film speeds are available even forinexpensive cameras, it is desirable that the rate of charging thecapacitor 16 be adjustable to accommodate such variations in film speed.In this circuit, the control over the charging rate of the cacpacitor 16is effective by adjusting the base bias on the transistor 12 by means ofthe adjustable resistor 20.

The shutter means itself may take any of a number of forms all of whichare responsive to the signal generated by the controlwmeans 10. In itssimplest form suggested in FIG. 2 the shutter means 8 is a fine wire ofvaporizable material positioned adjacent the flash bulb. When the wireis energized with the pulse of current from the control means 10, thesubstance of the wire vaporizes, depositing a thin layer of thesubstance of the wire on the external surface of the flash bulb 8. Thatthin layer deposited upon the surface of the flashbulb 6 acts as ashutter, operating under controlled conditions to block a furthertransmission of light from the flashbulb 6 toward the object of thephotograph.

In FIG. 3, the shutter means 8 is shown as a vaporiz able wire arrangedwith several turns of the wire wrapped about the external surface of theflashbulb 6. The added turns of the wire would provide a more uniformdistribution of the deposit of the vaporized substance of the wire overthe external surface of the flashbulb 6.

In FIG. 4, the shutter is shown as a vaporizable wire positioned withinthe envelope of the flashbulb 6. With this arrangement, actuation of theshutter results in a deposit of the vaporized substance of the wire onthe inside surface of the flashbulb, also effectively cutting off theflow of light toward the object.

A preferred form of the shutter is illustrated in FIG. 5 wherein theshutter member '8 comprises a normally transparent member positionedbetween the flash bulb 6 and the object of the photograph, and includingmeans for rendering the normally transparent member substantiallyopaque. The shutter member 8, in this instance, comprises a first andsecond transparent sheet member 34 defining two opposite walls of aclosed chamber. These transparent sheet members 34 may be formed of asuitable clear plastic. The remaining walls 36 of the structure definedin the closed chamber may be made of any suitable nonconductivematerial. Within the chamber thus defined and along one of thesidewalls, preferably the bottom, there is positioned a pyrotechnicelement 36. This pyrotechnic element may again be a vaporizable wiremember. However, in a preferred form, the pyro technic element is adeflagrating substance or a combination of a deflagrating substance anda dispersible substance. In each of these devices the integration of thesignal from the light sensitive element commences with the actuation ofthe camera shutter. Asthe integrated signal reaches a predeterminedvalue, the pyrotechnic shutter is actuated to prevent the remaininglight output from the flashbulb from influencing the photograph.

This principle is illustated in FIG. 6. FIG. 6 is a graph showing timein milliseconds as abscissa and relative intensity of the light asordinate. In a typical flash bulb, the light output builds upinintensity, from a time about two and a half milliseconds afteractuation, to peak at about 14 milliseconds, and then a progressivedecay to about 42 /2 milliseconds. Thus, the light output of theflashbulb covers a period of about 40 milliseconds. This characteristicis shown in the larger partially dotted curve. In the illustration shownin FIG. 6, the control signal produced as 'a result of the integrationof the reflected light is assumed to have initiated the actuation ofsufficient to actuate the the shutter at time milliseconds. Within onemillisecond, the effective lightoutput from the flash bulb has beensubstantially completely blocked. Although the incendiary orincandescent element in the flash bulb is not extinguished, theremaining portion of the light under the dotted portion of the curve isineffective to illuminate the object of the photograph.

. FIGS. 7 through illustrate the various steps in the production of apreferred embodiment of the present invention. This preferred embodimentfeatures, as hereinbefore mentioned, a pyrotechnic element 36 whichincludes a combination of a defiagrating substance and a dispersionsubstance such as that described in connection with FIG. 5. Beginningwith the pyrotechnic element, the base structure is anannular disc orbutton 38 made of a suitable ceramic material. This disc or button maybe on the order of 0.150 inch in diameter and on the order of 0.040 inchthick. A portion of the thickness adjacent one of the fiat faces of thedisc is of smaller diameter than the other portion, the smaller diameterbeing of the order of 0.120 inch. This produces a button having twosubsantially cylindrical concentric surfaces. The button also isprovided with a central aperture 44 on the order of 0.030 inch indiameter. By silk screen technique, an annulus of resist material 46 isdeposited on the larger of the two fiat surfaces. Next, a similarannulus 48 of resist material is deposited, through silk screentechniques, to the opposite flat surface of the button.

After the two annuli have been deposited, the discs or buttons 38 aresubjected'to one or more plating processes. Preferably, the buttons aresubjected to a first plating process which is the technique known aselectroless plating. Next they are subjected to a second plating processsuch as electro-plating. This plating process produces an electricallyconductive shell entirely surrounding the ceramic disc except for thoseareas where the resist annuli are positioned. Subsequent to the platingprocess the resist material is removed from the disc 38 by a suitablesolvent and the central aperture 44 is plugged, as by solder. Theresultant of the process thus far is a button having two electricallyisolated conductive surfaces 50 and 52 respectively. As may be seen mostclearly in FIGS. 11 and 14, the conductive surface 50 extends around theouter portion of the button with respect to the two annular grooves 54and 56, respectively, left by the removal of the resist material; whilethe conductive surfaces 52 is plated through the hole or aperture 44 andextending over the two flat surfaces to the inner edge of the groovesdefined by the removal of the resist material. These two conductivesurfaces comprise a pair of electrodes or contacts for the pyrotechnicelement 36.

An incandescent bridge member is secured to the larger face of thebutton, bridging the groove 54. This incandescent bridge may be either asmall piece of wire, such as Nichrome, 0.001 inch in diameter welded tothe two electrodes 50 and 52, as shown in FIG. 11, or it may be appliedby vacuum depositing techniques such as sputtering illustrated in FIGS.9 and 10. FIG. 10 is an enlarged fragmentary portion of the structureshown in FIG. 9. In FIG. 9 a vacuum chamber is represented by a bell jar60. Withinthe vacuum chamber there is positioned a firstsputtering-electrode 62 spaced from and parallel to a second electrode64. The second electrode is in the form of a holder for a number of theplated discs. These discs are positioned in the holder electrode withthe larger flat surface facing the electrode 62. Between the twoelectrodes, and closely adjacent the holder electrode 64 there ispositioned an apertured mask 66 having a plurality of small longitudinalapertures 68 therethrough. These apertures are each dimensioned tocorrespond to the size of the desired bridge member 58. The aperturescorrespond in number and position with the discs held in the holderelectrode 64. When the sputtering apparatus is actuated,

metal from the sputtering electrode 62 is deposited through theapertures 68 onto the surface of the disc or buttons 38 in a position tobridge the groove 54.

After the bridge 58 has been deposited, the holder electrode is removedfrom the vacuum chamber 60 and the mask 66 is removed from the face ofthe discs 38. The discs 38 however, are left in the holder 64 andsubjected to a further silk screen deposition process. In this step ofthe preparation,'a small quantity of defiagrating material is depositeddirectly over the incandescent bridge 58. In FIGS. 13 and 14, there isshown a deposit of such defiagrating material 70. In a preferredconstruction, the deflagrating material was lead styphnate. Thedeposited defiagrating material 70 is then dried.

From a thin sheet of suitable thermoplastic material, a first bubble isformed to constitute the outer shell of a capsule member. In FIG. 12there is shown such an outer bubble 72 having a domed portion 74, ashoulder portion 76 and a skirt portion 78. In a similar manner a secondor inner bubble or shell 80 is formed in a second sheet of plasticmaterial. The inner bubble is also formed with a dome portion, ashoulder portion and a skirt portion. However, the domed portion of theinner bubble is substantially shallower than the domed portion of theouter bubble, Similarly, the shoulder and skirt portion of the innerbubble are dimensioned to fit within the shoulder and skirt portion ofthe outer bubble. The outer bubble is placed in a suitable die 82, aquantity of dispersion material 84 is deposited in the outer bubble, andthe inner bubble is superimposed in the second bubble and the shoulderand skirt portions of the two bubbles are sealed together, therebyencapsulating the dispersion material between the two domed portions ofthe bubbles. The dispersion material may comprise comminuted graphite,lamp black or other finely comminuted substances.

The capsules or pods thus formed are then placed in another die 86. Thediscs 38, prepared as hereinbefore set forth, are inserted in thecapsules or pods with the larger fiat surface, that surface bearing thedeflagrating substance, resting on the shoulder portion of the capsules,the defiagrating substance itself lying within the chamber defined bythe surface of the inner bubble or shell and the flat surface of thedisc. In this position, the skirt portion of the capsule is folded overthe edge of the larger cylindrical portion of the disc or button 38 andsealed to the button as shown in FIG. 13.

The resultant of the process thus far described is a completepyrotechnic element 36 as shown in FIG. 14. The exposed portion of theconductive surface 50 becomes one electrical contact while the otherexposed conductive surface 52 becomes a second electrical contact, thesetwo contacts being electrically isolated from each other by the groove56. This pyrotechnic element assembly may now be secured in a suitablerecess in one of the side walls of the structure 8 shown in FIG. 5, andsecured in place with the contacts exposed on the outside of the chamberdefining means.

Referring now to FIG. 2, it may be seen that one of the contacts may beelectrically connected to one of the terminals 33 while the othercontact may be electrically connected to the other of the terminals 33.When, as hereinbefore described, the SCR is triggered into conduction,the capacitor 32 is discharged through the incandescing bridge member58. The incandescence of the bridge member 58 ignites the defiagratingsubstance 70. The defiagration explosively ruptures the incapsulatingbubbles 72 and 80 causing the dispersion material 84 to be scatteredwithin the chamber defined by the transparent sheet members 34. Thisdispersion of the material 84 deposits a pyrotechnic residue on theinside surfaces of the two transparent sheet members 34 substantiallyblocking the transmission of light therethrough. It has been found inapparatus constructed in accordance with the hereindescribed structure,that the dispersed material oating within the chamber also contributes,supposedly y diifusion characteristics, to the obliteration of theassage of light through the shutter member 8.

Since many of the cameras designed for use by tyro 'hotographers areadapted for use with the multrple/ lement flash devices known as flashcubes, there is shown 1 FIG. 15, an embodiment of the inventioncorrespond- ]g to the flash cube concept. In eifect, what is provided, ifour of the shutter mechanisms such as that shown in IG. 5 arranged inunique unitary structure. That unitary tructure comprises a circularflat cap member 88, a ylindrical body portion 90 and a flat circularbase memer 92. Coextensive with and supported within the cylinrical bodyportion is a tube member 94 which is subtantially square in crosssection. The square tube is upported from the inner wall of thecylindrical body tortion at each of the four corners of the square byupport means of the type generally referred to as a pider. The entirestructure of the body member includug the cylindrical portion, thesquare tube, and the pider may be made as one integral part as byextrusion holding of a clear plastic. The end capand base member nayalso be stamped from plastic, clear or otherwise. *our chambers aredefined between the inner wall of the ylindrical member and the outerwall of the square tube. Each of these chambers corresponds to thechamber deined between the transparent members 34 of the shutter means 8shown in FIG. 5.The cap member is sealed to me end of the body member90. Thebase member 92 is trovided with a central aperture 96 which issquare and limensionally matches the inner dimension of the square ube94. Adjacent each of the straight sides of the square .perture, one ofthe pyrotechnic elements 36 is secured o the base member 92 with thecontact members 50 and i2 being exposed on the opposite side of the basememer. With these pyrotechnic elements in place, the base nember issealed to the opposite end of the body portion rom the end cap 88, thuscompletely sealing the four :eripheral chambers. Thus arranged, one ofthe pyroechnic elements will be positioned within each of the ourperipheral members. The central tube 94, however, s open at the endadjacent the pyrotechnic elements '6. The cross sectional dimension ofthe square tube is lesignated to be complementary of the dimension ofthe :onventional flash cube. With the structure shown in FIG. .5 it isapparent that a conventional flash cube may be nserted within the squaretube portion of the structure ind if so inserted, would have one of thechambers to :onstitute the light obliterating shutter for each of the'our faces of the flash cube. Conversely, the substance )f the flashcube may be built into the inner square tube vith the inner plastic tube94 comprising the plastic vindows of the cube itself.

Thus, it may be seen, that there has been provided, n accordance withthe present invention, an improved :hotoflash control means whichautomatically controls he amount of light emanating from flash bulb toillumirate the object of a photograph after a time interval de- Lerminedby the demand of the conditions.

The embodiments of the invention in which an ex- :lusive property orprivilege is claimed are defined as folows:

1. In a photofiash system wherein an object to be Jhotographed isilluminated by a flash bulb, the improvenent comprising shutter meansin'the form of a pyro- :echnic device positioned between said flash bulband ;aid object, and means responsive to a predetermined imount of lightreflected from said object to actuate said thutter, said shutter beingoperable to interrupt the flow )f light from said flash bulb toward saidobject.

2. The invention as set forth in claim 1 wherein said pyrotechnic devicecomprises a vaporizable metallic elenent positioned to deposit, whenactuated, a light obscuring coating on the surface of the envelope ofsaid flash bulb.

3. The invention as set forth in claim 1 wherein said shutter meansincludes a normally transparent member positioned in the light pathbetween flash bulb and said object, and wherein-said pyrotechnic devicecomprises means rendering said transparent member non-transparent.

4. The invention as set forth in claim 3 wherein said pyrotechnic devicecomprises a vaporizable metallic element positioned to deposit, whenactuated, a light obscuring coating on the surface of said transparentmember.

5. The invention as set forth. in claim 3 wherein said pyrotechnicdevice comprises a deflagrating element positioned to deposit, whenactuated, a light obscuring residue on the surface of said transparentmember. v

6. The invention as set forth in claim 3 wherein said transparent membercomprises a first and a second transparent element defining two oppositefaces of a closed chamber, and wherein said pyrotechnic device ispositioned between said transparent elements and includes means fordischarging, when actuated, a pyrotechnic residue throughout saidchamber substantially blocking the passage oflight therethrough.

7. The invention as set forth in claim 6 whereinsaid pyrotechnic deviceincludes a deflagrating element and a pod of dispersible comminutedparticles, said pod being sealed over said deflagrating elementwherebysaid' deflagrating element when actuated, ruptures said pod,dispersing said particles into said chamber as said pyrotechnic residue.

8. The invention as set forth in claim 7 wherein said means responsiveto light includes a photoelectric element and an electric control signalgenerating circuit, said pyrotechnic device being responsive to acontrol signal generated by said circuit for actuation thereof.

'9. In a photoflash system wherein an object to be photographed isilluminated by light emitted from a source of flash light, theimprovement comprising shutter means in the form of a pyrotechnic devicepositioned adjacent to said source of flash light, and means responsiveto a predetermined amount of light reflected from said object to actuatesaid shutter means, said shutter means being actuable by saidlast-mentioned means to interrupt the flow of light from said sourcetoward said object.

10. In a photoflash system as set forth in claim 9 and including a shellhaving at least a normally transparent portion and encompassing saidsource and said pyrotechnic device and wherein said pyrotechnic deviceincludes a vaporizableelement positioned to deposit, when actuated bysaid last-mentioned means, a light blocking coating on an inside surfaceof said shell between said source and said object.

References Cited The following references, cited by the Examiner, are ofrecord in the patented file of this patent or the original patent.

UNITED STATES PATENTS Rentschler et a1. 43193 X JAMES W. LAWRENCE,Primary Eiraminer D. C. NELMS, Assistant Examiner US. Cl. X.R.

